On Fri, 9 Nov 2012, Jason J. Herne wrote: > Hello, I've noticed Linux seems to have issues with external USB > enclosures containing drives > 2 TB. The USB mass storage driver > apparently emulates a SCSI device and sends a read/write(10) for all Actually, it's the SCSI disk driver which decides whether to use the 10-byte or 16-byte commands. > requests where the target sector is not large enough to require the > use of read(16). The issue is that both of the enclosures I was able > to test with do not properly accept this read(10) command when a 3TB > drive (Seagate ST3000DM) is installed. A wireshark USB trace shows > the read(10) command is sent to the drive and the drive never > responds. The same drive/enclosure used in Windows 7 shows that > read(16) is always used to communicate with the drive and the drive > works without issue with Windows 7. > > A few notable points: > 1. The drive operates without issue when connected to an internal Sata > port on my computer. > 2. The enclosure operates without issue with Linux (using read(10) ) > when a <2TB drive is installed. > 3. I have reproduced this problem on several computers, and using > several Linux kernels, including 3.7.0-rc4-next. > > I first reported this to the linux-usb list thinking it was a USB > issue. At the request of Alan Stern I am also reporting it here. > > I'm not sure if I am taking the correct approach but I have written a > simple patch that resolves the problem for me. In essence, the patch > adds a new bit to scsi_device that indicates it should favor > read/write(16) over read/write(10). sd_prep_fn is modified to check > this bit, and use a 16 byte operations when a large capacity drive is > in use and this bit is on. Finally, the USB storage driver's scsi glue > code is modified to turn this bit on for all of its devices. Not sure > if this is the correct fix, but it works for me. I'm willing to It's not correct, because it will break USB drives that don't recognize the 16-byte commands. The bit should be set only for disks that have 2^32 or more blocks, or maybe only for disks that hold 2^41 or more bytes (and maybe only for USB disks). Since usb-storage doesn't know the disk's capacity, the decision about whether to set the new bit has to be made in the SCSI disk driver. Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
